A nine-year effort to resolve a mystery about the behaviour of neutrinos — particles that interact only weakly with matter — has thrown up an unexpected signal. The findings, announced on 11 April, leave open the possibility that new physics hides within the observations, but winnows the options for what that might be.

“The plot keeps thickening,” says Bill Louis, co-spokesperson for the MiniBooNE neutrino detector based at Fermilab in Batavia, Illinois.

MiniBooNE aimed to settle controversy raised by the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory in New Mexico, which looked at how one type of neutrino could turn into another, or 'oscillate'. The LSND findings suggested that a fourth kind of neutrino existed — a sterile neutrino. But many scientists were sceptical of the result.

Using a 12-metre sphere filled with 800 tonnes of mineral oil to catch neutrinos, the MiniBooNE team found no evidence, at high neutrino energies, for the sort of oscillation that workers at LSND had reported (A. A. Aguilar-Arevalo et al. preprint at http://arxiv.org/abs/0704.1500; 2007). “My view is that there is no longer any credible evidence for sterile neutrinos,” says Gary Feldman, a physicist at Harvard University. Team members say they are still working on the data and can't be so sure.

At low neutrino energies the detector saw more electron neutrinos in the experiment's beam of muon neutrinos than expected. The team can't yet explain the observation and the community's curiosity is piqued. “Already theorists are sending us their papers and saying look, we fit you,” says project co-spokesperson Janet Conrad.

Meanwhile, MiniBooNE has switched to observing antineutrinos, which is what LSND studied, to find out whether the variations between the two experiments' results are due to surprising differences in the behaviour of matter and antimatter.